PCM is a well-known method of transmitting telephone speech which consists of periodically sampling the amplitude of voice-frequency signal and translating these amplitudes into digital form. This method is well suited to the transmission of several voice channels using time division multiplex.
PCM transmission of telephone speech is extensively used in North America and in the world. Several schemes have been proposed for the purpose of reducing the bit rate of PC speech: in particular, the technique known as NIC (Nearly Instantenous Companding, invented by Deutweiller et Messerschmitt and described in U.S. Pat. No. 3,945,002 issued Mar. 16, 1976) which reduces from 8 bits per sample to 6 bits per sample with the provision that three overhead bits be sent every N samples (where N is some fixed integer between 6 and 128).